Optical signal transmission and transportation is a key enabling force in today's high speed digital communication infrastructure which supports vast amount of data transportation that are essential for many data centric informational applications such as, for example, internet application. With ever increasing demand for transportation bandwidth, new optical signal transmission and transportation systems are constantly being developed which trend toward higher data rate and higher channel density count.
Optical signal, in a format of binary or multi digital level, usually experiences certain amount of distortion during transportation that, together with other causes such as noise, affects overall system performance. Generally, the higher the data rate of and the longer a distance travelled by an optical signal, the bigger the amount of distortion that the optical signal usually experiences. Among many factors contributing to the optical signal distortion, chromatic dispersion of the transportation media such as fiber is a main factor. The amount of dispersion that an optical signal is able to tolerate in a transmission system varies inversely proportional to the square of the data-rate. As a general rule of thumb, for a 40 Gb/s direct detection system, the dispersion window is typically less than the equivalent of 10 km of SMF-28 fiber at 1550 nm wavelength.
FIG. 1 is a simplified functional block diagram of an optical signal transportation system with line protection scheme as is known in the art. Under normal working conditions, optical signals are usually transported over working fiber paths 31 and 32, as a bidirectional optical transportation system 1, between terminal 10 and terminal 20. If there is a fault such as fiber cut in one or both of the working fiber paths 31 and 32, the amount of optical signal received at photo-detector PD2 in terminal 10 and/or at photo-detector PD5 in terminal 20 will generally decrease to a level below a pre-defined threshold. As a result, this decrease in signal level triggers optical line protection (OLP) switches, such as SW1 and SW2 in terminal 10 and SW3 and SW4 in terminal 20, to switch and cause the system to transmit and receive optical signals via protection fiber paths 41 and 42 instead of working fiber paths 31 and 32. The same event, such as fiber cut, may also generate a system alarm to alert the happening and existence of such a fault in the working fiber paths 31 and 32. Optical signals transported over protection fiber paths 41 and 42 may continue to be monitored by photo-detectors PD3 and PD6. In the bidirectional transportation system 1 of FIG. 1, photo-detectors PD1 and PD4 are used to monitor the optical signal levels launched into the fibers, in both directions.
However, the above optical system configuration may not work well on fiber links with a narrow dispersion window due to difference in total fiber dispersion between the working fiber paths 31/32 and the protection fiber paths 41/42. This is especially true in a DQPSK system where data rate of the optical signal is high around 40 Gb/s or higher. Generally, in the above system in order to expand dispersion window such that an optical signal may be able to tolerate, fiber-bragg gratings (FBG) and more frequently Etalon-based channelized tunable dispersion compensation modules (TDCM), both of which are not shown in FIG. 1, are used at the receiving end of each channels of their respective terminals.
In order to get the optical transmission system back to work or be recovered, the tunable dispersion compensation module (TDCM) in each receiving channel is required to change or modify their dispersion setting so as to compensate any difference in the amount of dispersion between the working and the protection fiber paths. However, dispersion of this channelized TDCM is normally tuned through gradual temperature change which is usually considered as slow in the range of seconds, if not in the tens of seconds. Together with the process of using forward error correction (FEC) feedback or other feedback mechanism to find the right setting for the TDCM, the entire process of recovering the transmission system from fiber cut, for example, for just one channel may take several seconds and sometimes close to tens of seconds. It is known in the industry that for dynamic line protection application it is generally required that the system recovery time must be less than 50 ms. Obviously, the thermally-tuned TDCM is unable to meet the 50 ms recovery time requirements for the protection scheme of an optical transport system.